Oxide particle dispersion-strengthened ni-base superalloy

ABSTRACT

An oxide particle dispersion-strengthened Ni-base superalloy includes Ni, 0.1% by weight to 14.0% by weight of Ru, 0.1% by weight to 14.0% by weight of Al, and inevitable impurities and has a crystal structure containing 0.01% by weight to 3.0% by weight of dispersed oxide particles based on the total amount of the superalloy.

TECHNICAL FIELD

The present invention relates to an oxide particledispersion-strengthened Ni-base superalloy.

BACKGROUND ART

Components to be exposed to high-temperature environments, such asaircraft engines and power generation gas turbines, are required to bemade of materials having high mechanical properties and high oxidationresistance in a wide temperature range from room temperature to hightemperatures. Examples of such materials used include superalloys suchas Ni-base superalloys. Even now, there has still been a demand for thedevelopment of materials with a higher allowable temperature because thethermal efficiency of gas turbine equipment is required to be furtherimproved.

In general, Ni-base superalloys exhibit excellent properties based onsolid solution strengthening and y′ phase precipitation strengtheningmechanisms. Many excellent single crystal casting alloys have alreadybeen developed based on such strengthening mechanisms. However, theallowable temperature improvement based on these strengtheningmechanisms gradually becomes difficult with increasing temperature. Onthe other hand, oxide particle dispersion-strengthened Ni-basesuperalloys are promising materials because it is considered that theirallowable temperature can be improved based on an oxide fine particledispersion strengthening mechanism in addition to the abovestrengthening mechanisms.

Oxide particle dispersion-strengthened Ni-base superalloys have acharacteristic structure in which a large number of oxide fine particlestypically with sizes of 1 μm or less are dispersed in the matrix phasewhere elements other than Ni form a solid solution in Ni. Depending onalloy composition, they can also have a structure in which a precipitatesuch as the y′ phase is also dispersed in addition to the oxideparticles.

Oxide particle dispersion-strengthened Ni-base superalloys developed sofar include MA6000 alloys (Patent Literatures 1 to 3 and Non PatentLiterature 1) and TMO-2 alloys (Patent Literatures 4 to 5 and Non PatentLiterature 2). Basically, these alloys are produced by a process thatincludes preparing an alloy powder by mechanical alloying and thenconsolidating the alloy powder by hot extrusion or other processes.TMO-2 alloys have a higher level of high-temperature strength becausethey have a tungsten (W) or tantalum (Ta) content higher than that ofMA6000 alloys. Other oxide particle dispersion-strengthened Ni-basesuperalloys have also been proposed (Patent Literature 6).High-temperature corrosion and high-temperature oxidation of an yttriaparticle dispersion-strengthened alloy have also been examined andreported (Non Patent Literature 3).

CITATION LIST Patent Literatures

Patent Literature 1: US 3,926,568

Patent Literature 2: JP 49-74616 A

Patent Literature 3: US 4,386,976

Patent Literature 4: JP 62-99433 A

Patent Literature 5: US 4,717,435

Patent Literature 6: JP 63-53232 A

Non Patent Literature

Non Patent Literature 1: G. A. J. Hack, “Oxide ParticleDispersion-Strengthened Superalloy,” Denki-Seiko, 57 (1986), 341

Non Patent Literature 2: Yozo Kawasaki, Katsuyuki Kusunoki, ShizuoNakazawa, Michio Yamazaki, “Development of TMO-2 Alloy,”Tetsu-to-Hagane, 75 (1989), 529-536

Non Patent Literature 3: Isao Tomizuka, Yutaka Koizumi, Shizuo Nakazawa,Hideo Numata, Katsumi Ono, Akimitsu Miyazaki, “High-TemperatureCorrosion and High-Temperature Oxidation of Yttria ParticleDispersion-Strengthened Alloy,” Zairyo-to-Kankyo, 42 (1993), 514-520

SUMMARY OF INVENTION Technical Problem

As mentioned above, many types of oxide particle dispersion-strengthenedNi-base superalloys have been developed so far. However, a sharp rise inenergy prices and an increase in energy demand have created a demand forfurther improvement of the thermal efficiency of gas turbine equipment.Now, the development of materials with a higher allowable temperature isdemanded in order to achieve further improvement of thermal efficiency.

Although having superior high-temperature strength, the TMO-2 alloyscontains 12% by weight or more of W so as to have improvedhigh-temperature strength. Such a high W content is effective inimproving mechanical properties but may reduce high-temperaturecorrosion resistance.

It is therefore an object of the present invention to provide a noveloxide particle dispersion-strengthened Ni-base superalloy that can havesuperior high-temperature corrosion resistance and a higher level ofmechanical properties such as high-temperature strength.

Solution to Problem

To solve the problem, the inventors have investigated the composition ofoxide particle dispersion-strengthened Ni-base superalloys. As a result,the inventors have found that when having a ruthenium (Ru) content inthe range of 0.1% by weight to 14.0% by weight, oxide particledispersion-strengthened Ni-base superalloys can have goodhigh-temperature corrosion resistance and a higher level of mechanicalproperties such as high-temperature strength. The present invention hasbeen accomplished based on such findings.

That is, an oxide particle dispersion-strengthened Ni-base superalloy ofthe present invention has a composition including Ni, 0.1% by weight to14.0% by weight of Ru, 0.1% by weight to 14.0% by weight of aluminum(Al), and inevitable impurities, the superalloy also has a crystalstructure containing 0.01% by weight to 3.0% by weight of dispersedoxide particles based on the total amount of the superalloy.

Advantageous Effects of Invention

The oxide particle dispersion-strengthened Ni-base superalloy of thepresent invention containing 0.1% by weight to 14.0% by weight of Ru canhave good high-temperature corrosion resistance and a significantlyimproved level of high-temperature strength and other mechanicalproperties, such as a significantly improved level of mechanicalproperties after heating at a temperature in the range of 1,260° C. to1,300° C. as shown in the examples below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a chart showing the result of X-ray diffraction of an extrudedmaterial according to an example of the present invention in comparisonwith that of a mechanical alloying powder.

FIG. 2 is a photograph of the structure of an extruded materialaccording to an example of the present invention, which is observed witha transmission electron microscope.

FIG. 3 is a graph showing plots of micro Vickers hardness versusannealing temperature for a comparison between Examples 1 to 3 of thepresent invention and Comparative Example after annealing.

DESCRIPTION OF EMBODIMENTS

As described above, an oxide particle dispersion-strengthened Ni-basesuperalloy of the present invention has a composition including Ni, 0.1%by weight to 14.0% by weight of Ru, 0.1% by weight to 14.0% by weight ofaluminum (Al) inevitable impurities, the superalloy also has a crystalstructure containing 0.01% by weight to 3.0% by weight of dispersedoxide particles based on the total amount of the superalloy.

Such an oxide particle dispersion-strengthened Ni-base superalloy of thepresent invention may also preferably include 0.1% by weight to 14.0% byweight of Ru, 0.1% by weight to 14.0% by weight of Al, and at least oneof 0.1% by weight to 14.0% by weight of Re, 0.1% by weight to 20.0% byweight of Co, 0.1% by weight to 20.0% by weight of Cr, 0.1% by weight to15.0% by weight of Mo, 0.1% by weight to 20.0% by weight of W, 0.1% byweight to 10.0% by weight of Ti, 0.1% by weight to 10.0% by weight ofNb, 0.1% by weight to 15.0% by weight of Ta, 0.01% by weight to 10.0% byweight of Hf, 0.01% by weight to 10.0% by weight of Zr, 0.1% by weightto 5.0% by weight of V, 0.1% by weight to 10.0% by weight of Pt, 0.1% byweight to 10.0% by weight of Pd, 0.1% by weight to 10.0% by weight ofIr, 0.001% by weight to 1.0% by weight of B, or 0.001% by weight to 1.0%by weight of C.

Ru, which is one of the elements characteristic of the presentinvention, can form a solid solution in the y phase as the matrix phaseand increase the high-temperature strength by solid solutionstrengthening. Ru can suppress the precipitation of the TCP phase, whichcan form when Re or other elements are added, so that Ru can increasethe high-temperature strength. The Ru content is preferably in the rangeof 0.1 to 14.0% by weight, more preferably in the range of 1.0 to 14.0%by weight. If the Ru content is less than 0.1% by weight, the TCP phasewould precipitate at high temperatures, which can make it impossible toensure a high level of high-temperature strength and thus is notpreferred. On the other hand, if the Ru content is more than 14% byweight, the c phase would precipitate to reduce the high-temperaturestrength, which is not preferred. The base metal price of Ru is about200 to 300 times higher than that of Ni or other metals. Therefore, theRu content is preferably as low as possible in the range where solidsolution strengthening can be produced to increase the high-temperaturestrength. Economically, the Ru content preferably has an upper limit of8.0% by weight.

The addition of Al can cause the y′ phase precipitation and thuscontribute to the improvement of strength by precipitationstrengthening. The Al content is preferably in the range of 0.1 to 14.0%by weight. If the Al content is less than 0.1% by weight, theprecipitation strengthening would be insufficient, which can make itimpossible to ensure the desired high-temperature strength and thus isnot preferred. If the Al content is more than 14.0% by weight, a largeamount of a coarse y′ phase would be formed to degrade the mechanicalproperties.

The content of oxide particles added for dispersion strengthening shouldbe 0.01% by weight to 3.0% by weight. The oxide particles may be of anytype. In particular, the oxide particles are preferably of yttriumoxide, which has high chemical stability at high temperatures. Whenhigh-purity yttrium oxide is added, however, some elements in the alloymay react with yttrium oxide to form a complex oxide during manufactureor high-temperature use. Therefore, a complex oxide of yttrium oxide andaluminum oxide, such as Y₄Al₂O₉, is also preferably used instead ofyttrium oxide.

The type and content of elements other than Ru and Al are not restrictedand may be controlled depending on the intended use or desiredproperties. Depending on the intended use, other alloying elements maypreferably include 0.1% by weight to 14.0% by weight of Re, 0.1% byweight to 20.0% by weight of Co, 0.1% by weight to 20.0% by weight ofCr, 0.1% by weight to 15.0% by weight of Mo, 0.1% by weight to 20.0% byweight of W, 0.1% by weight to 10.0% by weight of Ti, 0.1% by weight to10.0% by weight of Nb, 0.1% by weight to 15.0% by weight of Ta, 0.01% byweight to 10.0% by weight of Hf, 0.01% by weight to 10.0% by weight ofZr, 0.1% by weight to 5.0% by weight of V, 0.1% by weight to 10.0% byweight of Pt, 0.1% by weight to 10.0% by weight of Pd, 0.1% by weight to10.0% by weight of Ir, 0.001% by weight to 1.0% by weight of B, or0.001% by weight to 1.0% by weight of C.

A more preferred composition (% by weight) may be as follows.

Ru: 1.0-8.0

Al: 1.0-10.0

Cr: 1.0-10.0

Co: 1.0-10.0

Mo: 0.1-4.0

W: 1.0-8.0

Ta: 1.0-10.0

Hf: 0.05-5.0

Zr: 0.05-5.0

Ti: 0.1-5.0

Nb: 0.1-5.0

Re: 0.1-8.0

V: 0.1-2.0

Pt: 0.1-6.0

Pd: 0.1-6.0

Ir: 0.1-6.0

B: 0.005-0.05

C: 0.005-0.05

Oxide particles: 0.1-3.0

Re can form a solid solution in the y phase as the matrix phase andincrease the high-temperature strength by solid solution strengthening.Re can also be effective in improving corrosion resistance. However, theaddition of a large amount of Re may cause the TCP phase to precipitateas a harmful phase at high temperatures and thus reduce thehigh-temperature strength. For such addition of Re, the Re content ismore preferably in the range of 0.1 to 14.0% by weight.

If the Re content is less than 0.1% by weight, the solid solutionstrengthening of the y phase may be insufficient, which can make itimpossible to ensure the desired high-temperature strength and thus isnot preferred. If the Re content is more than 14.0% by weight, the TCPphase may precipitate at high temperatures, which can make it impossibleto ensure a high level of high-temperature strength and thus is notpreferred.

Cr is an element highly resistant to oxidation and can increase thehigh-temperature corrosion resistance. If the Cr content is less than0.1% by weight, the high-temperature corrosion resistance can fail to beachieved. A Cr content of more than 20.0% by weight can suppress the y′phase precipitation and cause the production of a harmful phase such asthe a or p phase, which can reduce the high-temperature strength andthus is not preferred.

Co can increase the solid solution limit of Al, Ta or other elements inthe matrix phase at high temperatures and can cause the y′ phaseprecipitation and thus increase the high-temperature strength. If the Cocontent is less than 0.1% by weight, the y′ phase precipitation can beinsufficient, which can make it impossible to increase thehigh-temperature strength. If the Co content is more than 20.0% byweight, the balance between Co and any of other elements such as Al, Ta,Mo, W, Hf, and Cr can be broken, which can cause the precipitation of aharmful phase and the reduction of the high-temperature strength andthus is not preferred.

Mo can form a solid solution in the y phase as the matrix phase whencoexisting with W and Ta so that Mo can increase the high-temperaturestrength and contribute to the high-temperature strength byprecipitation hardening. If the Mo content is less than 0.1% by weight,these effects can be insufficient, which can make it impossible toincrease the high-temperature strength. A Mo content of more than 15.0%by weight can reduce the high-temperature corrosion resistance and thusis not preferred.

W can increase the high-temperature strength by the action of solidsolution strengthening and precipitation hardening when coexisting withMo and Ta. If the W content is less than 0.1% by weight, the formationof the solid solution in the y and y′ phases can be insufficient, whichcan make it impossible to increase the high-temperature strength. The Wcontent is preferably 20.0% by weight or less. This is because a Wcontent of more than 20.0% by weight may reduce the high-temperaturecorrosion resistance.

Pt, Pd, and Ir can also form a solid solution in the y phase as thematrix phase and thus increase the high-temperature strength by solidsolution strengthening. In order to obtain this effect, the content ofeach of these elements should be at least 0.1% by weight. However, sincethe price of these elements, which belong to the platinum group metals,is about 500 to 3,000 times higher than that of Ni, the content of eachof these elements is preferably 10.0% by weight or less, more preferably6.0% by weight or less.

Ta, Ti, and Nb can contribute to precipitation strengthening bysubstituting for Al sites in the y′ phase. Ta, Ti, and Nb can alsoincrease the high-temperature strength by the action of solid solutionstrengthening and precipitation strengthening when coexisting with Moand W. If the Ta content is less than 0.1% by weight, these effects canfail to be achieved. The Ta content is preferably 15.0% by weight orless. This is because a Ta content of more than 15.0% by weight cancause the formation of the a or p phase and thus reduce thehigh-temperature strength. If the content of each of Ti and Nb is lessthan 0.1% by weight, it can be impossible to achieve precipitationstrengthening or solid solution strengthening in the co-presence of Moand W. The content of each of Ti and Nb is preferably 10.0% by weight orless. This is because if the Ti or Nb content is more than 10.0% byweight, a harmful phase may form to reduce the high-temperaturestrength.

V is an element that can form a solid solution in the y′ phase andstrengthen the y′ phase. If the V content is less than 0.1% by weight,these effects can fail to be achieved. The V content is preferably 5.0%by weight or more. This is because if the V content is more than 5.0% byweight, the creep strength may decrease.

Hf is a grain boundary segregating element which can segregate at thegrain boundary between the y and y′ phases to strengthen the grainboundary, so that it can increase the high-temperature strength. Toachieve these effects, the Hf content should be at least 0.01% byweight. If the Hf content is more than 10.0% by weight, local meltingcan occur, which may reduce the high-temperature strength and thus isnot preferred.

In addition to Hf, the same may apply to the addition of Zr.

B is a grain boundary strengthening element which can increase thehigh-temperature strength. To achieve these effects, the B contentshould be at least 0.001% by weight. If the B content is more than 1.0%by weight, a harmful carbide can precipitate at grain boundaries, whichis not preferred.

In addition to B, the same may apply to the addition of C.

More specifically, examples of the elemental composition of the oxideparticle dispersion-strengthened Ni-base superalloy of the presentinvention include the following as well as those of the examples shownbelow.

Ni-Ru-Al-Re-Co-Cr-Mo-W-Ta-Hf-oxide particles

Ni-Ru-Al-Re-Co-Cr-Mo-W-Ta-Hf-(B,C)-oxide particles

Ni-Ru-Al-Re-Co-Cr-Mo-W-Ta-(Ti,Nb)-(Hf,Zr)-(B,C)-oxide particles

Ni-Ru-Al-Re-Co-Cr-Mo-(W,V)-Ta-(Pt,Pd,lr)-(B,C)-oxide

Ni-Ru-Al-Re-Cr-(Mo,W,Co,V)-(Ta,Ti)-(B,C)-oxide particles

Ni-Ru-Al-Cr-(W,Co,V)-(Ta,Ti)-(B,C)-oxide particles

Ni-Ru-Al-Cr-(Ta,Ti)-oxide particles

There is no restriction on the method for producing the oxide particledispersion-strengthened Ni-base superalloy of the present invention. Ingeneral, a powder metallurgy technique may be used to uniformly dispersethe oxide particles. For example, the oxide particledispersion-strengthened Ni-base superalloy of the present invention canbe produced by a process that includes producing an alloy powder bymechanical alloying, then sealing the alloy powder in a case, and thenconsolidating the alloy powder by hot extrusion or other techniques.Alternatively, the alloy powder may be solidified by hot isostaticpressing (HIP), hot pressing, or other techniques. Alternatively, theoxide particle dispersion-strengthened Ni-base superalloy of the presentinvention may be produced by solidifying the alloy powder by any ofthese techniques and then subjecting the solidified product to hotextrusion or hot rolling.

In general, the combination of base and additive elements forconstituting a heat-resistant alloy is determined depending on theintended use of the alloy and cost effectiveness. For example, alloysfor gas turbines are required to have high-temperature strength in thetemperature range of 400 to 500° C., when they are for use in turbinedisks, or required to have high strength and high-temperature corrosionresistance in the temperature range of about 800 to about 1,000° C.,when they are for use in combustors, nozzles, turbine blades, shrouds,and other components.

In addition, finer oxide particles with a shorter particle-particledistance can increase the strengthening effect of the dispersedparticles, in other words, can increase the effect of preventingdislocation motion, which would otherwise allow the oxide particles tocause deformation. However, excessive addition of the dispersedparticles can make it difficult to perform deformation. In order for thedispersion-strengthened alloy to be workable and have an appropriatelevel of toughness, the dispersed particle size distribution ispreferably in the range of 0.001 μm to 5 μm, and thus the oxideparticles to be used preferably has a primary particle size distributionof 1 μm or less, taking into account particle aggregation during theproduction. The content of the oxide particles, for example, in theNi-base alloy for gas turbines is, in particular, preferably 0.5 to 3.0%based on the total amount of the alloy in order to obtainhigh-temperature strength at 800° C. or higher. The particle sizedistribution of the alloy or metal powder to be used may have an upperlimit of, for example, 250 μm, so that mechanical alloying and sinteringfor consolidation can be performed with improved efficiency.

Mechanical alloying becomes possible when in a high-energy ball mill,the impact energy between moving steel balls and between the vessel andmoving steel balls (that is mechanical energy) accumulates in the powderbetween them through the compression crushing and shear millingprocesses. In this case, the mixture particles are subjected to forgewelding and repeated folding, so that atomic-level alloying can occureven at a low temperature such as around room temperature due todiffusion. Successful alloying requires high impact energy, and alloyingefficiency should also be improved. For these purposes, the ratio of themixture power weight to the steel ball weight is preferably 1/10 to 1/20in an attritor and ⅕ to 1/10 in a planet ball mill, and the rotationalspeed of the ball mill is preferably 50 to 400 rpm although it dependson the ball mill size. The alloying is preferably performed for 20 hoursor more. In case of oxygen contamination during the alloying, purgingthe inside of the ball mill tank with an inert atmosphere such as argon(Ar) is preferably performed as a pre-alloying treatment.

The consolidation of the dispersed alloy-forming powder by sintering canbe performed by a powder metallurgy process that includes charging thepowder into a mild steel vessel and then subjecting the powder to hotextrusion or HIP. The sintering of the powder to form the Ni-base alloyis preferably performed in the temperature range of 1,000 to 1,300° C.taking into account diffusion, fusion, and densification between theparticles, the formation of more solid solution of alloy atoms, and thehigh-temperature stability limit of the oxide particles. In thisprocess, the inside of the vessel is subjected to a vacuum treatment asa pretreatment for the purpose of removing water, oxygen, and othercontaminants, which exist in or adsorb on the space in the vessel, theinner surface of the vessel, and the surface of the powder. The vacuumtreatment is preferably a heat treatment under a vacuum of 10⁻¹ to 10⁻³torr at a temperature of 100° C. to 600° C. for 10 minutes to 10 hoursdepending on the size of the facility so that oxygen can be prevented asmuch as possible from being added to the dispersion alloy and a strongoxide can be prevented from forming on the surface of the powder.

Example 1

An extruded material with a composition ofNi-6.4Co-4.5Cr-1.1Mo-4.0W-5.8Al-7.5Ta-0.1Hf-6.3Re-4.9Ru-1.1Y₂O₃ (eachnumerical value is in units of % by weight) was prepared as describedbelow. Raw material powders were mixed to reach the target compositionas a whole and then subjected to a mechanical alloying process using anattritor. After the process, the alloy powder was charged into a caseand subjected to a vacuum treatment, and the case was sealed. The alloypowder was then consolidated by hot isostatic pressing (HIP) at aheating temperature of 1,180° C. and a pressure of 118 MPa. Theresulting HIP product was subjected to hot extrusion under theconditions of a heating temperature of 1,200° C. and an extrusion ratioof 5 to form a round bar-shaped extruded material.

FIG. 1 is a chart showing the result of X-ray diffraction of theextruded material in comparison with that of the mechanical alloyingpowder. The X-ray diffraction chart (chart A) of the mechanical alloyingpowder mainly had the (111) and (200) diffraction peaks of the Ni solidsolution. In the case of the extruded material (chart B), the (110)diffraction peak of the y′ phase was observed in addition to thediffraction peaks of the Ni solid solution. This indicates that theextruded material is composed mainly of the Ni solid solution (y) phasein which the y′ phase is precipitated.

FIG. 2 shows the structure of the extruded material observed with atransmission electron microscope. In the drawing, the arrows indicateoxide particles. The structure was found to contain a large number offine oxide particles of several tens of nanometers dispersed in crystalgrains.

Small pieces were cut from the extruded material and then each subjectedto an isothermal heat treatment at a temperature of 1,260° C. to 1,300°C. for 1 hour. This heat treatment was performed taking into account thefact that in general, oxide particle dispersion-strengthened Ni-basesuperalloys are produced as hot extruded materials or hot rolledmaterials and then further subjected to hot forging or the like at suchtemperatures so that they can be worked into members with desired sizeand shape. After the heat treatment, the micro Vickers hardness of eachsample was 598 in the case of 1,260° C. and 585 in the case of 1,290° C.

The high-temperature corrosion resistance of the resulting alloy wasevaluated by a molten salt corrosion test. A cylindrical piece with adiameter of 6 mm and a height of 4.5 mm was cut from the extrudedmaterial in such a way that the axis direction coincided with theextrusion direction. The cut piece was immersed in a molten salt andheated at a temperature of 800° C. for 4 hours. The molten salt used asa corrosive medium was a 3:1 mixture of sodium sulfate and sodiumchloride. A crucible was charged with the mixed salt in such an amountthat the cylindrical sample could be completely immersed in the salt.The mixed salt was preheated at 800° C. in a heating furnace. After thetemperature was stabilized sufficiently, the cylindrical sample wasimmersed in the molten salt. The rate of reduction in the samplediameter between before and after the test was calculated to be 0.17%.

Example 2

An extruded material with a composition ofNi-5.9Co-3.8Cr-0.9Mo-3.9W-6.1Al-8.6Ta-0.2Hf-5.3Re-4.6Ru-1.2Y₄Al₂O₉ (eachnumerical value is in units of % by weight) was prepared as describedbelow. Raw material powders were mixed to reach the target compositionas a whole and then subjected to a mechanical alloying process using anattritor. After the process, the alloy powder was charged into a caseand subjected to a vacuum treatment, and the case was sealed. The alloypowder was then subjected to hot extrusion under the conditions of aheating temperature of 1,050° C. and an extrusion ratio of 15 to form around bar-shaped, extruded material.

Small pieces were cut from the extruded material and then each subjectedto an isothermal heat treatment at a temperature of 1,260° C. to 1,300°C. for 1 hour. After the heat treatment, the micro Vickers hardness ofeach sample was 626 in the case of 1,260° C. and 585 in the case of1,290° C.

The high-temperature corrosion resistance of the resulting alloy wasevaluated by the molten salt corrosion test under the same conditions asin Example 1. The sample was immersed in the molten salt and heated at atemperature of 800° C. for 4 hours. The rate of reduction in the samplediameter between before and after the test was calculated to be 0.17%.

Example 3

An extruded material with a composition ofNi-6.1Co-3.8Cr-0.9Mo-4.2W-6.3Al-9.2Ta-0.2Hf-5.0Re-4.7Ru-1.2Y₄Al₂O₉ (eachnumerical value is in units of % by weight) was prepared as describedbelow. Raw material powders were mixed to reach the target compositionas a whole and then subjected to a mechanical alloying process using anattritor. After the process, the alloy powder was charged into a caseand subjected to a vacuum treatment, and the case was sealed. The alloypowder was then subjected to hot extrusion under the conditions of aheating temperature of 1,050° C. and an extrusion ratio of 15 to form around bar-shaped, extruded material.

Small pieces were cut from the extruded material and then each subjectedto an isothermal heat treatment at a temperature of 1,260° C. to 1,300°C. for 1 hour. After the heat treatment, the micro Vickers hardness ofeach sample was 664 in the case of 1,260° C. and 596 in the case of1,290° C.

The high-temperature corrosion resistance of the resulting alloy wasevaluated by the molten salt corrosion test under the same conditions asin Examples 1 and 2. The sample was immersed in the molten salt andheated at a temperature of 800° C. for 4 hours. The rate of reduction inthe sample diameter between before and after the test was calculated tobe 0.17%.

Comparative Example

Non Patent Literature 2 shows the results of the micro Vickers hardnesstest of an extruded TMO-2 alloy material after an isothermal heattreatment. In this test, the extruded material has an alloy compositionofNi-9.8Co-5.9Cr-2.0Mo-12.4W-4.2Al-4.7Ta-0.8Ti-0.05Zr-0.05C-0.01B-1.1Y₂O₃(each numerical value is in units of % by weight). It should be notedthat this alloy has a W content about three times higher than that ofthe alloy composition of the examples.

Non Patent Literature 3 shows the results of the molten salt corrosiontest of this alloy under the same conditions as in Examples 1 to 3. Thisextruded material shows a diameter reduction rate of 29.0%.

FIG. 3 is a graph showing plots of micro Vickers hardness versusannealing temperature for a comparison between the materials of Examples1 to 3 and Comparative Example after the isothermal annealing (1 hour).Open circles represent the results of Example 1, triangles those ofExample 2, squares those of Example 3, and solid circles those ofComparative Example. The graph shows that although having asignificantly reduced W content, the oxide particledispersion-strengthened Ni-base superalloy of the present invention hasimproved mechanical properties after heating at a temperature in therange of 1,260° C. to 1,300° C. because it contains Ru.

As a result of the molten salt corrosion test, Examples 1 to 3 all showa diameter reduction rate of 0.17% whereas Example shows a diameterreduction rate of 29.0%, which indicates that Examples 1 to 3 have asignificantly improved level of high-temperature corrosion resistance.This would be because the alloys of Examples 1 to 3 have a W contentsignificantly lower than that of the alloy of Example.

1. An oxide particle dispersion-strengthened Ni-base superalloy having acomposition comprising Ni, 0.1% by weight to 14.0% by weight of Ru, 0.1%by weight to 14.0% by weight of Al, and inevitable impurities, thesuperalloy also having a crystal structure containing 0.01% by weight to3.0% by weight of dispersed oxide particles based on the total amount ofthe superalloy.
 2. The oxide particle dispersion-strengthened Ni-basesuperalloy according to claim 1, which comprises 0.1% by weight to 14.0%by weight of Ru, 0.1% by weight to 14.0% by weight of Al, and at leastone of 0.1% by weight to 14.0% by weight of Re, 0.1% by weight to 20.0%by weight of Co, 0.1% by weight to 20.0% by weight of Cr, 0.1% by weightto 15.0% by weight of Mo, 0.1% by weight to 20.0% by weight of W, 0.1%by weight to 10.0% by weight of Ti, 0.1% by weight to 10.0% by weight ofNb, 0.1% by weight to 15.0% by weight of Ta, 0.01% by weight to 10.0% byweight of Hf, 0.01% by weight to 10.0% by weight of Zr, 0.1% by weightto 5.0% by weight of V, 0.1% by weight to 10.0% by weight of Pt, 0.1% byweight to 10.0% by weight of Pd, 0.1% by weight to 10.0% by weight ofIr, 0.001% by weight to 1.0% by weight of B, or 0.001% by weight to 1.0%by weight of C.